Compact heating module with soft start

ABSTRACT

An electronic heating module includes a heating transistor operated in its linear region as a current source. The temperature of the heating transistor is detected and used to control whether the heating transistor is turned on or off. Further control of the heating transistor is provided via a feedback control loop that monitors a voltage at one terminal of the heating transistor.

FIELD OF THE INVENTION

[0001] The present invention relates to an electrical heating apparatus.More specifically, the present invention relates to a compact heatingmodule having a soft start feature.

BACKGROUND OF THE INVENTION

[0002] A conventional electrical heating system typically comprises aheating element and a separate current source. Commonly, a temperaturecontrol circuit is also used to control the temperature of the heatingelement. With the use of separate components, however, the conventionalsystem is not cost-effective to manufacture and results in inefficientutilization of generated heat.

[0003] It is thus desirable to provide a compact heating module thatregulates the temperature of a heating element by controlling a currentsupplied to the heating element with fewer circuit components, and whichutilizes heat generated by the components efficiently.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to provide a compactheating module that regulates a temperature of a heating element bycontrolling a current supplied to the heating element with few circuitcomponents, and which utilizes heat generated by the componentsefficiently.

[0005] The above and other objectives are achieved according to theinvention by the provision of a compact heating module comprising: apower supply; a transistor coupled between the power supply and groundfor operation in a linear-region to generate heat; a temperaturedetecting unit thermally coupled to the heating transistor to detect atemperature of the heating transistor; and a control unit coupled to thepower supply and to the temperature detecting unit to apply a controlvoltage to the heating transistor to turn the heating transistor on oroff based on the detected temperature.

[0006] According to a preferred embodiment of the invention, the controlunit may comprise a first resistive element having a first node coupledto the power supply and a second node coupled to the temperaturedetecting unit and the heating transistor. The heating module mayfurther comprise a second resistive element coupled between the heatingtransistor and ground, a third resistive element having a firstelectrode coupled to the second resistive element, and anothertransistor having its control electrode coupled to a second electrode ofthe third resistive element and having a main current conductingelectrode coupled to a control electrode of the heating transistor. Theheating module may further comprise a capacitive element having a firstelectrode coupled to a control electrode of the heating transistor forreceiving the control voltage. Preferably, the temperature detectingunit used in the heating module may be a thermistor.

[0007] In a further embodiment of the invention, the heating module maybe equipped with means for moving air across the heating transistor. Forexample, a fan may be used to force air across the heating transistor oracross a heat sink thermally coupled to the heating transistor. In sodoing, heat is efficiently distributed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention is better understood by reading the followingdetailed description with reference to the accompanying figures, inwhich like reference numerals refer to like elements throughout, and inwhich:

[0009]FIG. 1 is a circuit schematic showing an exemplary illustration ofa heating module according to an embodiment of the present invention;and

[0010]FIG. 2 shows a further example of a block diagram of a heatingmodule according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011]FIG. 1 illustrates an example of a heating module according to anexemplary embodiment of the present invention. The constant currentsource of the heating module includes a heating transistor Q-1. Theheating transistor Q-1 is used as both a heating element and acontrolling element to control the amount of heat generated. The heatingtransistor Q-1 of the present invention may be of any suitable type, forexample, a BJT or a FET, and may be arranged on a heat sink (shown, forexample, in FIG. 2). The conductivity type of the transistor shown inFIG. 1 may be changed, and the same overall function of the circuit canbe maintained, for example, by reversing the polarity of the powersupply. The heating transistor Q-1 has a control electrode Q-1 c andfirst and second electrodes Q-1 a and Q-1 b. The DC power supply may beof any reasonable magnitude for a heating transistor, e.g., 5V or 12V.The first and second electrodes Q-1 a and Q-1 b are coupled between theDC power supply and ground to enable current flow through thetransistor. The heating transistor Q-1 may also be coupled to aresistive element, which in turn is connected to the ground. Theresistance of the resistive element may be, for example, about 0.1Ω. Theresistive element may be formed by any single resistor or anycombination of resistors or resistive elements whose overall resistanceprovides the desired resistance value. For example, FIG. 1 shows theresistive element being comprised of parallel connected resistiveelements R-4 and R-5, which may, for example, comprise twoparallel-connected 0.1Ω, 3 W resistors.

[0012] The constant current source also includes a feedback controlcircuit. The feedback control is provided via a second transistor Q-2.The second transistor Q-2 may be of any type, for example, a BJT or aFET, and typically includes first and second electrodes Q-2 a and Q-2 band a control electrode Q-2 c. A resistive element R-3 is connectedbetween the control electrode Q-2 c of the second transistor Q-2 and anintervening node N-1 between the heating transistor Q-1 and theresistive elements R-4 and R-5. The first and second electrodes Q-2 aand Q-2 b are coupled between the control electrode Q-1 c of the heatingtransistor Q-1 and ground. The resistive element R-3 may be of about100Ω, such that the voltage at the control electrode Q-2 c of the secondtransistor Q-2 may be about 0.6V when the second transistor Q-2 isturned on.

[0013] A voltage divider comprising a resistive element R-1 and athermistor T-1 produces an output to control the turn-on and turn-off ofheating transistor Q-1 and, in turn, the current flow through heatingtransistor Q-1. The thermistor T-1 is in thermal contact with theheating transistor Q-1 to detect the temperature thereof. The thermistorT-1 may be a negative-temperature-coefficient (NTC) type, in whichresistance decreases as temperature rises. R-1 is chosen in accordancewith the resistance of thermistor T-1 and in accordance with desiredfunctionality, as will become apparent from the discussion below. Anoutput of the voltage divider is coupled to the control electrode Q-1 cof the heating transistor Q-1 via a resistive element R-2. The resistiveelement R-2 may be of about 47 kΩ.

[0014] The heating module may be provided with a soft start feature.When a soft start feature is provided, the control gate Q-1 c of theheating transistor Q-1 is coupled to a capacitive element C-1 to“soften” (i.e., make gradual, rather than abrupt) the initial turning-onof the heating transistor Q-1 during a power-up of the heating module.The capacitive element C-1 may be a 16V capacitor with a capacitance ofabout 1 μF.

[0015] One of ordinary skill in the art would appreciate that thediscussed values of resistive and capacitive components are exemplaryand can be adjusted depending on design needs and the particularenvironments in which the heating module is going to be used. Moreover,the thermistor T-1 may also be changed to apositive-temperature-coefficient type by making accompanying changes tothe rest of the circuitry in FIG. 1. The conductivity type of the secondtransistor Q-2 may be changed from one type (e.g., NPN) to another type(e.g., PNP) with accompanying changes to the rest of the circuitry inFIG. 1.

[0016] An example of the operation of the above-described heating moduleis now provided. When power is provided from a power supply to theheating module during a power-up of the heating module, the voltagedivider R-1 and T-1 divides the power supply voltage based on a ratio ofthe resistances therein and provides the divided voltage to the controlelectrode Q-1 c of the heating transistor Q-1, via the resistive elementR-2. The capacitive element C-1 acts as a short circuit when the dividedvoltage is first applied to the control electrode and slowly raises thevoltage at the control electrode Q-1 c of the heating transistor Q-1 togradually turn on the transistor. When the heating transistor Q-1 isturned on, it operates in a linear region, where the transistor gain issmall compared to its saturation region and a large amount of heat isgenerated because of the inefficiency in transistor gain. Most of theheat generated by the heating module comes from this linear operatingregion of the heating transistor Q-1.

[0017] After the heating transistor Q-1 is turned on, the secondtransistor Q-2 turns on in response to a rise in the voltage applied toits control electrode Q-2 c via resistive elements R-3, R-4 and R-5.When the second transistor Q-2 turns on, it initially lowers the voltageat the control electrode Q-1 c of the heating transistor Q-1 to slightlyreduce the current through the heating transistor Q-1. However, thecurrent is quickly stabilized at a constant value by virtue of theoperation of a feedback loop formed via the heating transistor Q-1 andtransistor Q-2. The constant current through the heating transistor Q-1keeps heating transistor Q-1 in its linear region to generate heat,which may then be drawn off, for example, using a heat sink, andutilized. The temperature of the heating transistor Q-1 increasesgradually because of its operation in the linear region. As temperaturegradually increases, the resistance of the thermistor T-1 graduallydecreases. When the temperature of the heating transistor exceeds apredetermined threshold temperature, the resistance of the thermistorfalls below a predetermined resistance threshold, which corresponds tothe temperature threshold. When the resistance of the thermistor T-1drops below the resistance threshold, the output of the voltage dividerbecomes low enough to turn the heating transistor Q-1 off. After thetransistor Q-1 turns off, the second transistor Q-2, in turn, turns off.Consequently, there is no current flow through the heating transistorQ-1, and its temperature slowly drops. When the temperature of theheating transistor Q-1 drops below the predetermined thresholdtemperature by a certain predetermined temperature margin, which may bechosen to be any number of degrees or zero, as desired, the resistanceof the thermistor T-1 increases, and the output of the voltage dividerturns the heating transistor Q-1 on again. Hence, the temperature of theheating transistor Q-1 is regulated to be within a certain margin of thepredetermined temperature.

[0018] In the present embodiment, by using the heating transistor Q-1 asboth a heating element and a current source, the number of componentsrequired in the heating module is reduced, and any heat generated by thecurrent source is utilized to generate the heat output. Thus, theheating module of the present invention is efficient and cost-effective.

[0019]FIG. 2 shows a further embodiment of the invention, in blockdiagram form. Heating transistor 1 is shown, along with its associatedcircuitry (e.g., as shown in FIG. 1) 3. Heating transistor 1 is shownthermally coupled to a heat sink 2; however, heat sink 2 may be omitted,with an attendant loss in efficiency. FIG. 2 further shows air movingmeans 4, which moves air across heat sink 2, from which heat fromheating transistor 1 is radiated. When air moving means 4 moves airacross heat sink 2, the air is heated by heat radiated from heat sink 2and may then be directed as desired. Note that air moving means 4 maycomprise a fan or any other suitable means, including passive means, formoving air across a heat source.

[0020] The embodiments illustrated and discussed in this specificationare intended only to teach those skilled in the art the best way knownto the inventors to make and use the invention. Nothing in thisspecification should be considered as limiting the scope of the presentinvention. The above-described embodiments of the invention may bemodified or varied, and elements added or amended, without departingfrom the invention as appreciated by those skilled in the art in lightof the above teachings. It is thus to be understood, within the scope ofthe claims and their equivalence, the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. An electronic heating module comprising: aheating transistor having terminals coupled to a power supply and toground and configured to operate as a current source; a temperaturedetecting unit arranged to detect a temperature of the heatingtransistor; and a control unit arranged to receive a signal from thetemperature detecting unit and to provide a control voltage to a controlelectrode of the heating transistor to cause the heating transistor toalternately operate in a linear operating region and to not operate,depending on the detected temperature.
 2. The electronic heating moduleaccording to claim 1, further comprising: a soft-start means that causesthe control voltage provided from the control unit to gradually approacha value that causes the heating transistor to operate in its linearoperating region.
 3. The electronic heating module according to claim 2,wherein the soft-start means comprises: a capacitive element coupledbetween the control electrode of the heating transistor and ground. 4.The electronic heating module according to claim 1, wherein the controlunit comprises: a first resistive element having a first electrodecoupled to the power supply and a second electrode coupled to both thetemperature detecting unit and the control electrode of the heatingtransistor.
 5. The electronic heating module according to claim 1,wherein the control unit includes: a second resistive element coupledbetween the heating transistor and ground; a third resistive elementhaving a first electrode coupled to the second resistive element; and acontrol transistor having a control electrode coupled to a secondelectrode of the third resistive element and having a main electrodecoupled to the control electrode of the heating transistor.
 6. Theelectronic heating module according to claim 5, wherein the controltransistor comprises a BJT.
 7. The electronic heating module accordingto claim 5, wherein the control transistor comprises a FET.
 8. Theelectronic heating module according to claim 1, wherein the temperaturedetecting unit comprises a thermistor.
 9. The electronic heating moduleaccording to claim 8, wherein the control means includes a firstresistive element coupled to the power supply and to the thermistor,thereby forming a voltage divider.
 10. The electronic heating moduleaccording to claim 1, wherein the heating transistor comprises a FET.11. The electronic heating module according to claim 1, wherein theheating transistor comprises a BJT.
 12. The electronic heating moduleaccording to claim 1, further comprising: a heat sink coupled to theheating transistor for drawing off heat from the heating transistor. 13.The electronic heating module according to claim 1, further comprising:air moving means arranged to move air across the heating transistor tothereby absorb heat from the heating transistor.
 14. The electronicheating module according to claim 13, wherein the air moving meanscomprises a fan.
 15. An electronic heating module comprising: a heatingtransistor having terminals coupled to a power supply and to ground andconfigured to operate as a current source; a temperature detecting unitarranged to detect a temperature of the heating transistor; a controlunit arranged to receive a signal from the temperature detecting unitand to provide a control voltage to a control electrode of the heatingtransistor so as to cause it to alternately operate in a linearoperating region and to not operate, depending on the detectedtemperature; air moving means arranged to move air across the heatingtransistor to thereby absorb heat from the heating transistor; and asoft-start means that causes the control voltage provided from thecontrol unit to gradually approach a value that causes the heatingtransistor to operate in its linear operating region.
 16. A method ofcontrolling a temperature of an electronic heating module comprising thesteps of: operating a heating transistor in a linear region of thetransistor; detecting a temperature of the heating transistor; andapplying a control voltage to a control electrode of the heatingtransistor to turn the heating transistor on or off based on thedetected temperature.
 17. The method according to claim 16, wherein thestep of applying a control voltage comprises a step of dividing a powersupply voltage based on the detected temperature and supplying thedivided voltage to the control electrode of the heating transistor. 18.The method according to claim 16, further comprising a step of operatingthe heating transistor as a current source.
 19. The method according toclaim 16, further comprising a step of preventing an abrupt rise of thecontrol voltage when the heating transistor is turned on during apower-up of the heating module.
 20. The method according to claim 16,further comprising a step of causing air to flow over the heatingtransistor and to thereby absorb heat from the heating transistor. 21.The method according to claim 16, wherein the step of applying a controlvoltage comprises a step of applying a feedback control voltage to thecontrol node of the heating transistor.